Atraumatic vascular graft removal sheath

ABSTRACT

The present invention comprises articles and methods for atraumatic removal of a chronically implanted medical device, such as a vascular graft. Specifically, the invention comprises a thin, lubricious and durable tubular cover that aids in protecting the indwelling implant during implantation while also acting as an atraumatic removal aid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 12/397,886, filed Mar. 4, 2009, which isincorporated by reference herein in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to atraumatic removal of chronicallyimplanted medical devices.

BACKGROUND OF THE INVENTION

It is sometimes necessary to extract chronically implanted medicaldevices, particularly endocardial leads for use with pacemakers andimplantable cardioverter-defibrillators (ICDs), if such leads fail orshould infection develop.

Removal of such implanted devices presents several obstacles. Forexample, fibrous tissue growth along any portion of a cardiac lead mayhamper extraction and can lead to trauma in adjacent cardiovasculartissues.

Various methods have been developed to remove chronically implanteddevices. For cardiac leads, a common removal method is traction, whereina longitudinal force is applied to the lead body after exteriorizing theproximal end of the lead. However, complications and difficultiesencountered when using traction have prompted development of surgicalapproaches and intravascular counterpressure and countertraction (Loveet al. PACE 2000 23:544-551).

Surgical approaches involve exposing the heart and great veins viasternotomy or thoracotomy followed by extraction of the lead via atransmural incision in the atrium or ventricle. While this removaltechnique is generally successful, it requires skills not generallyavailable and is associated with morbidity and high cost (Love et al.PACE 2000 23:544-551).

In cardiovascular countertraction, a sheath made of polymer or metalslides distally over the lead body. A locking stylet may be passedthrough the interior of the lead and locked at the lead tip to localizetraction forces at the tip. Traction applied to the lead pulls ingrowntissue to the sheath. Traction force is countered by the tipcircumference of the sheath which applies a localized shear stress onthe fibrotic tissue, separating it from the lead body. (Ellenbogen,Kenneth A., and others, ed. Clinical Cardiac Pacing, Defibrillation, andResynchronization Therapy (Philadelphia, Pa. Saunders Elsevier, 2007),885-886.)

Counterpressure is a similar extraction technique used when calcifiedtissues are present in which countertraction force is converted topressure localized between the edge of the sheath and adjacent tissues.This pressure peels calcified masses from the vein and/or heart walls,thus damaging the tissues (Ellenbogen, 2007, 886).

Alternative sheaths include those using powered, mechanical cuttingtips, RF ablation and laser vaporization. (Ellenbogen, 2007, 886-890.)However, each method is associated with significant risks. Thereforethere is a need for an alternative and safer method of extractingmedical devices from a patient.

SUMMARY OF THE INVENTION

The present invention provides for a safer and less traumatic device andmethods for removing a chronically implanted device in a patient. Oneembodiment of the invention provides for a medical device comprising animplantable diagnostic or therapeutic lead having a distal end, aproximal end, a longitudinal axis and an outer surface; and a tubularcover attached to the diagnostic or therapeutic lead, preferably nearthe distal end, and positioned to cover a substantial portion of theouter surface of the diagnostic or therapeutic lead. The tubular coveris configured to evert upon application of a longitudinal force toextract the diagnostic or therapeutic lead.

The present invention also provides for an evertable tubular coversurrounding portions of a lead, whereby portions of said lead will notbe covered. In another embodiment, a lead will be comprised of severaltypes of tubular covers, wherein said covers are evertable when the leadis extracted following implantation. In another embodiment, said tubularcover will tear and evert from specific sections where there is tissueattachment, tissue ingrowth or another type of constraint that causessaid tubular cover to tear as a lead is extracted. In anotherembodiment, said tear away tubular cover comprises a means of permittingtearing in specific locations along the longitudinal axis of saidtubular cover. In one embodiment, said means comprise perforations beinggenerally oriented circumferentially around the tubular cover. Inanother embodiment, said perforations are oriented in helical patternaround the tubular cover. In another embodiment, said means compriseweakened areas of material that will tear circumferentially and evert asa lead is extracted.

The present invention also provides a tubular cover for attachment to adiagnostic or therapeutic lead configured to allow energy to pass fromthe lead and into adjacent fluids or tissue. Alternatively, the lead maybe configured to allow energy to pass from adjacent fluids or tissueinto the lead, for example in the case of sensing by cardiac pacingleads. The tubular cover may be attached to at least the distal end ofthe lead and positioned to cover a substantial portion of the outersurface of the lead. The tubular cover is configured to evert uponapplication of a longitudinal force to extract the lead. Where the leadconducts electrical energy into adjacent fluids or tissues, the tubularcover allows both such energy transfer and gases generated byelectrolysis to pass into adjacent fluids. This avoids damage to thelead or the cover. Where energy is conducted from surrounding fluids andtissues into the lead, the tubular cover also allows such transfer.

The present invention also provides for a medical device comprising atubular cover, said tubular cover configured to evert upon applicationof a longitudinal force to the medical device following implantation. Inone embodiment, said medical device is chronically implanted in ananimal. In another embodiment, said medical device is prone to tissueincorporation. In another embodiment, said tubular cover comprisesePTFE. In another embodiment, wherein said medical device is selectedfrom the group consisting of lead generators, arteriovenous (A-V) accesscatheters, peripherally inserted central (PICC) catheter lines, venousfeeding catheters, feeding tubes, breathing tubes, and implanted sensingdevices.

The present invention also provides methods for making and extraction ofdiagnostic or therapeutic leads with tubular covers of the inventionwith minimal damage to surrounding tissues.

The present invention also provides for a medical device comprising avascular graft, a tubular cover positioned over a substantial portion ofthe outer surface of the vascular graft, wherein said tubular cover isconfigured to evert upon application of a longitudinal force to thestent-graft following implantation.

Another embodiment of the invention comprises a tubular cover for avascular graft, said tubular cover comprising a material configured intoa tubular cover positionable over a substantial portion of an outersurface of a vascular graft and will evert upon application of alongitudinal force. In another embodiment, said vascular graft comprisesa stent. In another embodiment, said vascular graft comprises ePTFE. Inanother embodiment, the tubular cover comprises ePTFE. In anotherembodiment, said ePTFE comprises at least one additive.

The present invention also provides for a vascular graft having a lengthextending between opposing ends comprising, a first tubular elementextending for the length of the vascular graft, a first stent coaxiallyaffixed around a first length portion of the first tubular element atone of said opposing ends, a second tubular element coaxially affixedaround a second length portion of the first tubular element that doesnot include a stent, wherein the length of the second portion is greaterthan the length of the first length portion; and a tubular coverpositioned over a substantial portion of the outer surface of thevascular graft, wherein said tubular cover is configured to evert uponapplication of a longitudinal force to the stent-graft followingimplantation. In one embodiment, one end of said second tubular elementis located adjacent to one end of said first stent. In anotherembodiment, said first tubular element provides an uninterrupted luminalsurface extending for the length of the vascular graft. In anotherembodiment, said first stent is a self-expanding stent. In anotherembodiment, said vascular graft comprises a therapeutic agent. Inanother embodiment, said therapeutic agent comprises herparin. Inanother embodiment, said heparin is on the luminal surface of saidvascular graft. In another embodiment, said vascular graft comprisesePTFE.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical cardiac pacing and ICD lead.

FIG. 2 shows a device of the present invention with an implantable leadand tubular cover.

FIG. 3 is a cross section taken at A-A in FIG. 2.

FIG. 4 is a diagram of the apparatus used to test and measure withdrawalforce for a sample device.

FIG. 5 is a close-up of the mandrel of the apparatus of FIG. 4.

FIGS. 6A and 6B shows attachment of a tubular cover of the presentinvention, which everts upon removal, over a mandrel for measurement ofwithdrawal force as described in Example 3.

FIG. 7A through 7C are diagrams showing attachment of a double walledtubular cover attached to a mandrel for measurement of withdrawal forceas described in Example 4.

FIG. 8 is a diagram of the apparatus used to test for electricalconductance and release of electrolysis-produced gases.

FIGS. 9A and 9B depict an alternative embodiment of the tubular cover ofthe invention.

FIG. 10 depicts a perspective view of a stented vascular graft with thetubular cover of the invention.

FIGS. 11A and 11B is a longitudinal cross section of stented vasculargraft with a tubular cover. FIG. 11B further shows a longitudinal crosssection of a stented vascular graft with a tubular cover in a bodyconduit.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention will hereinafter be described in connectionwith the preferred embodiments and methods of use thereof, it will beunderstood that it is not intended to limit the invention to theseembodiments and methods of use. Instead, it is intended to cover allalternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as described and claimed.

The present invention solves a long felt need in the medical field,providing an implantable medical device which can be removed afterimplant without causing significant risk or complications, includingtrauma to the tissue in which said medical device was implanted or tothe surrounding tissue. This need is especially pronounced fordiagnostic or therapeutic leads.

In one embodiment, the medical device comprises a diagnostic ortherapeutic implantable transmission lead with an evertable tubularcover surrounding at least a portion the lead. The term “implantabletransmission lead” is meant to include an implantable lead constructhaving a length, a distal end, a proximal end, an outer surface and alongitudinal axis, wherein said lead is capable of delivering energy toor from the body or bodily fluids, typically in the form of electricalenergy, but may also include acoustic energy and electromagnetic energy,such as light. Such energy may be beneficial for a range of medicalapplications such as cardiac pacing and defibrillation, neurologicalsensing and stimulation, physiological sensing, diagnostic applications,therapeutic applications, signal transmission applications, or othersimilar functions a device implanted in a subject may deliver to orreceive from the body.

Alternatively, the device comprises an implantable transmission leadwith an evertable tubular cover surrounding at least a portion of thelead. In one embodiment, the cover is capable of allowing energy to passthrough the tubular cover (from the lead and into adjacent fluids ortissue). In another embodiment, said energy is electrical energy. Inanother embodiment, said tubular cover allows gases generated byelectrolysis to pass through the cover and into adjacent fluids. Forexample, a typical defibrillation electrode out-gasses and formsundesirable bubbles during rapid, repeated energy pulses. Bubbleformation at an electrode is described by G H Bardy et al. Circulation73, 525 538, March 1986. The formation of bubbles at the electrodedegrades the energy waveform. Excessive bubble formation can result inincreased conduction resistance, which raises the energy required fordefibrillation and increases local current density. Thus, for a leadwhich transmits electrical energy, it would be desirable to have gasand/or bubbles (comprised of said gas) to diffuse through the tubularcover.

By “implantable” it is meant that the device is suited for embeddinginto a body of an animal, e.g. a human, surgically (i.e. via anincision) or other methods known in the art. In one embodiment, animplantable lead is chronically implanted in an animal. “Chronicallyimplanted” is considered to include, but is not limited to, long termimplantation (e.g. at least 29 days) and/or at least a period of timesufficient to generate tissue growth into (tissueingrowth/incorporation) and/or tissue adhesion by tissue growth onto(tissue attachment/adhesion) an implanted medical device.

By “evert”, “evertable”, “everted” it is meant that the tubular coverfolds onto itself such that an outer surface becomes an inner surface,or conversely an inner surface becomes an outer surface. See, forexample, FIGS. 6A and B and FIG. 11B.

Referring to the drawings in which like reference numbers represent likeor corresponding elements in the drawings.

FIG. 1 illustrates an implantable lead 100 of the conventional bipolarpacing and ICD type. The implantable lead 100 has a length 101, a distalend 102, a proximal end 103, an outer surface 104 and a longitudinalaxis 105 which are coaxial with the implantable lead. The lead has oneor more proximal connectors 180 for connecting to a power source andoperating system (not shown). A lead body 170 extends toward the distalend 102. Two electrodes 150 and 160 serve as needed to defibrillate theheart. The pacing and sensing circuit is formed by pacing electrode 110which, in this case, is shaped as a moveable helical coil and sensingring 140. The operating system sends electrical pulses for pacing andreceives electrical cardiac signals through the pacing and sensingcircuit.

FIG. 2 illustrates the implantable lead 100 and a tubular cover 190 ofthe present invention. The tubular cover 190 has a length 191, typicallyin the range of 40-100 cm, a distal end 192, a proximal end 193, anouter surface 194 and a longitudinal axis 105 centrally coaxial with theimplantable lead. Typical leads have diameters of about 1.0 to about12.0 French, or even larger, depending on intended function and implantlocation. The tubular cover is sized to be positioned over a substantialportion of the outer surface of the implantable lead. The term“substantial portion” refers to a covering positioned over at leastabout 50% of the length of the lead body 170 of said implantable lead.In another embodiment, the term “substantial portion” refers to acovering positioned over at least about 60%, about 70%, about 80%, about90%, about 95%, about 100% of the length of the lead body 170 of saidimplantable lead. “Substantial portion” also includes a non-continuouscovering that is positioned over a lead in segments, e.g., betweenelectrodes or on the electrodes (140, 150, 160), thereby leaving aportion of lead body lead uncovered. FIGS. 9A and 9B illustrate anexample of a non-continuous covering of an implantable lead.

FIG. 3 illustrates a cross section at “A-A” in FIG. 2. Tubular cover 190surrounds an outer insulative layer 200 surrounding an outer coiledconductor 210. The inside of the conductor is lined with anotherinsulative layer 220 which covers an inner coiled conductor 230. Aninsulative liner 240 with a lumen 250 is positioned adjacent innerconductor 230.

The tubular cover comprises a sheath-like structure that can bepositioned over a substantial portion of the outer surface of animplantable lead. The tubular cover may be attached to at least thedistal end of said lead by mechanical, rail or interference fit,mechanical structures, heat bonding or by a biocompatible adhesive orother securing means. Example adhesives are thermoplasticfluoropolymers, such as fluorinated ethylene propylene (FEP). Thetubular cover may also be formed as an integral part of the implantablelead at manufacture, thereby not requiring any attachment. The tubularcover may also be attached to an implantable device following itsmanufacture as a separate component. In one embodiment, the tubularcover has “double walled” construction wherein the cover is folded backon itself. Thus, when a lead comprising an evertable “double walled”tubular cover is extracted, the cover will slide over itself, thusreducing the force necessary to extract the lead.

In another embodiment, said tubular cover will tear at specific sectionswhere there is tissue attachment, tissue ingrowth or another type ofconstraint that causes said tubular cover to tear as a lead isextracted. Said tear away tubular cover comprises a means of permittingtearing in specific locations along the longitudinal axis of saidtubular cover. Said means will allow the portions of the tubular coverto tear circumferentially and evert as a lead is extracted. In oneembodiment, said means comprise perforations being generally orientedcircumferentially around the tubular cover. In another embodiment, saidperforations are oriented in helical pattern around the tubular cover.In another embodiment, said means comprise weakened areas of materialthat will tear circumferentially and evert as a lead is extracted. Inthese embodiments, said tubular cover will be secured (as describedabove) to the lead at strategic locations so that if said tubular covertears, said section will evert. In some embodiments, all the sectionswill tear and evert as said implantable lead is extracted. In otherembodiments, only a few sections will tear and evert as said implantablelead is extracted.

FIGS. 9A and 9B depict one of the embodiments described above. FIG. 9Adepicts a typical cardiac lead 900 comprising electrodes 902 and 904,distal tip housing 906, and fixation member 908. FIG. 9A furthercomprises tear away tubular covers 910A, 910B, and 910C which compriseperforations 912A, 9126, and 912C, respectfully. In this embodiment,said tear away tubular cover is applied in different sections of lead900, leaving electrodes 902 and 904 exposed. However, in otherembodiments, a tubular cover is applied to the whole lead.

FIG. 9B depicts a cross section of lead 900 in FIG. 9A as it isextracted from an animal, e.g. a human. As shown, fixation member 908 isretracted into tip housing 906 and an extraction force, as depicted inarrow 914, is applied to lead 900. Tubular cover 910A and 9106 is tornat perforations 912A and 912B (see FIG. 9A) and everts, as indicated byarrows 916A and 916B. As the lead 900 is extracted tubular cover 910Aand 910B will continue to evert until lead 900 is fully extracted and/orsaid tubular cover is fully everted. As described above, not allsections of said tubular cover will tear, as depicted by the intacttubular cover 910C. Note, that FIGS. 9A and 9B are only meant todemonstrate one embodiment of the invention and is not to be limiting inany way.

The tubular cover is dimensioned in diameter and length to cover thedesired portion of the implantable device. In one embodiment, if saidtubular cover has a “double walled” construction, then the amount ofmaterial used to make said tubular cover will be at least double thelength desired to cover said implantable device because said cover islocated over the lead and is folded back on itself. Preferred tubularcover thicknesses are about 0.25 mm or less, about 0.23 mm or less,about 0.20 mm or less, about 0.18 mm or less, about 0.15 mm or less,about 0.13 mm or less, about 0.10 mm or less, about 0.07 mm or less,about 0.06 mm or less, about 0.05 mm or less, about 0.04 mm or less,about 0.03 mm or less, about 0.02 mm or less, about 0.01 mm or less.Cover thickness can be determined by measurement of a transversecross-section of a covered electrode using an optical comparator, orother suitable means. The tubular cover is configured to evert uponapplication of a longitudinal force to remove the implantable lead.

In certain embodiments the tubular cover may be formed by longitudinallyor helically wrapped films. Various desired configurations may beachieved by varying film materials and characteristics (e.g., thicknessand width), wrapping parameters such as wrap angles and lap width, andfinished thickness of the cover. Alternatively, the tubular cover may beextruded, for example directly over a lead body or as a pre-manufactureditem subsequently attached to a lead.

The tubular cover provides for atraumatic removal of the devicefollowing implantation. Atraumatic removal connotes extraction orremoval of an implanted device by a means which reduces or decreasesdamaging shear forces to surrounding tissues associated with removal ofthe implanted device. A reduction or decrease is measured by comparisonto removal of a similar device without the tubular cover of theinvention. Without being bound to any particular theory, it is believedthat atraumatic removal of implantable devices is accomplished in thepresent device via the tubular cover which is everted upon removal thusreducing the force required for removal of the implantable device andminimizing the trauma and tissue damage associated with removal of suchdevices, e.g., traditional pacing and ICD leads. This force reduction isbelieved due to extraction forces being converted primarily into peelforce, which is localized at the distal end of the everting tubularcover and especially at the edge where the outer tube surface is beingeverted into an inner tube surface as the implantable device, to whichthe cover is attached, is being withdrawn. This differs from otherextraction techniques, such as traction, where extraction forces, suchas shear forces, may exist along a substantial length of the implantabledevice.

As demonstrated herein, use of a tubular cover in accordance with thepresent invention reduces the shear force needed to remove animplantable lead by at least 3-fold. See Table 1 below.

Said tubular cover can be constructed from any flexible biocompatiblematerial. Such material can be porous, non-porous, permeable, and/orimpermeable. Examples of such materials include, but are not limited to,silk, silicone, fluoropolymers such as expanded polytetrafluoroethylene(ePTFE), high density polyethylene (HDPE), and other polymers such aspolyesters and polyimides. As used herein, the term “porous” describes amaterial that contains small or microscopic openings, or pores. Withoutlimitation, “porous” is inclusive of materials that possess pores thatare observable under microscopic examination. “Non-porous” refers tomaterials that are substantially free of pores. The term “permeable”describes a material through which fluids (liquid and/or gas) canpenetrate or pass, particularly biological fluids. “Impermeable”describes materials that block the passage of fluids. It should beappreciated that a material may be non-porous yet still be permeable tocertain substances.

Said tubular cover is constructed by methods known in the art, e.g.extrusion, and will depend on the materials used to make said tubularcover. In one embodiment, the tubular cover is constructed of materialswhich inhibit tissue ingrowth which may further enhance evertability. Inanother embodiment, said materials may be impregnated, filled, imbibedor coated with at least one chemical compound known to inhibit tissueingrowth and/or deliver other clinical benefit, for example, chemicalcompounds that cause a bioactive response. Compounds that cause abioactive response comprise anti-microbials (e.g. anti-bacterials andanti-virals), anti-inflammatories (e.g. dexamethasone and prednisone),anti-proliferatives (e.g. taxol, paclitaxel and docetaxel) andanti-coagulating agents (e.g. heparin, abciximab, eptifibatide andtirofibran). In one embodiment, said anti-inflammatory is a steroid. Inanother embodiment, said steroid is dexamethasone. Said tubular covercan also be impregnated, filled, imbibed or coated with electricallyconductive materials, such as carbon, radio-opaque elements to fostervisualization during implantation and/or extraction and/or withmaterials which “lubricate” the cover, thus allowing the material toslide smoothly across itself. In another embodiment, said tubular covercan be coated and/or wetted immediately before implantation by thephysician and/or nurse and/or technician. In another embodiment, theinvention comprises a kit comprising a tubular cover and a wetting agentfor wetting said tubular cover. In another embodiment, the inventioncomprises a kit comprising a tubular cover, a medical device and awetting agent for wetting said tubular cover. Said wetting agent can beany agent herein above and/or a combination of agents described hereinand/or any agent that can be used to wet said tubular cover.

In another embodiment, the tubular cover can be constructed from amaterial which inhibits tissue (i.e. cellular) ingrowth and which isevertable upon application of a longitudinal force used to remove theimplantable device, thereby reducing the force required to remove thedevice. Although tissue ingrowth may not be a significant problem forremoving said implantable device in this embodiment, there can still betissue attachment to said tubular cover, which can cause tissue damagewhen said lead is extracted.

Expanded polytetrafluoroethylene is a preferred material for the tubularcover due to its thinness, strength, biocompatibility, lubricity,ability to evert in a tubular form, and upon appropriate treatment, tobecome “wettable” and thus electrically transmissive. EPTFE is a porousmaterial that is comprised of a thin, high strength, stretched,non-woven web of polytetrafluoroethylene composed substantially of nodesinterconnected by fibrils. In addition, ePTFE can be engineered to havea microstructure that can modulate the degree of tissueingrowth/incorporation and/or attachment/adhesion. In one embodiment,said tubular cover is made from ePTFE material that will not allowingrowth/incorporation and/or attachment/adhesion. In anotherembodiment, said ePTFE fibrils have mean length of less that about 3.0microns, about 1.0 microns and more preferably between about 0.05 andabout 0.4 microns. In this embodiment, ePTFE will not allow tissueingrowth/incorporation and/or attachment/adhesion. It is recognized thatthe above pore sizes are an average of a specific piece of material andthat some larger pores in the material may allow a minor amount ofcellular ingrowth/incorporation and/or attachment/adhesion. The poressizes, shapes and quantity can also be engineered to allow penetrationof conductive bodily fluids while restricting the ingrowth of tissue. Inanother embodiment, the ePTFE can have a mean fibril length thatencourages ingrowth/incorporation and/or attachment/adhesion (typicallyhaving a mean fibril length greater than about 6.0 microns). The meanfibril length of ePTFE is estimated by examination of scanning electronphotomicrographs of the surfaces of the particular film samples.

In another embodiment, said tubular cover is made from ePTFE which hasbeen engineered to encourage ingrowth/incorporation and/orattachment/adhesion in specific sections of the tubular cover, while, inother sections, said ePTFE (or other material) will not allow forcellular ingrowth and/or cellular adhesion. In this embodiment, themicrostructure of said ePTFE can be varied as a function of length alongthe tubular cover. For example, in some locations, at least one narrowband of material which encourages ingrowth/adhesion is placed on oralong a portion or segment of the tubular cover. Said narrow band can beplaced longitudinally along said medical device (either in segments oracross the entire length of the medical device) or circumferentiallyaround (in specific areas) said medical device. Said portion of thetubular cover that encourages ingrowth/incorporation and/orattachment/adhesion can help anchor the medical device in place and/orprovides a constraining force that ensures eversion will occur when saidmedical device is extracted. In another embodiment, said bands of ePTFEthat encourages ingrowth/incorporation and/or attachment/adhesion isgenerally located around the distal ends of tear away covers so as toprovide a constraining force that ensures that tearing and eversion willoccur when said medical device is extracted. In another embodiment, saidbands of ePTFE that encourages cellular ingrowth/adhesion is located inany area that ensures eversion when said medical device is extracted.

It may also be desirable to modify the ePTFE used for the presentinvention by incorporating various additives with said ePTFE. Fillerscan be incorporated in ePTFE by known methods, such as the methodstaught by U.S. Pat. No. 5,879,794, to Korleski. Additives can also beimbibed into the ePTFE by known methods. Additives can also be coated onthe ePTFE by known methods. Suitable additives include, for example,materials in particulate and/or fiber form and can be polymers,adhesives, elastomers, ceramics, metals, metalloids, carbon, andcombinations thereof. Particularly useful additives include, forexample, radiopaque materials, such as certain metals (e.g. bariumalloys) and carbon. The additives can be used in combination withdesired adhesive materials when incorporated with the polymer. It mayalso be desirable to metalize the ePTFE or at least a portion thereof.An additive may be included in the matrix of the polymer itself, orcontained within the voids defined by the polymeric structure, or both.Desirable fillers may also include colorants, medicaments,anti-microbials, antivirals, antibiotics, antibacterial agents,anti-inflammatory agents, anti-proliferative agents, anti-coagulatingagents, hemostatic agents, analgesics, elastomers and mixtures thereof.Compounds which lubricate an ePTFE cover, thus allowing the material toslide smoothly across itself, can be used to coat, fill, or imbibe thecover. Solid lubricants (i.e. graphite, waxes, silicone), fluidlubricants (i.e. hydrocarbon oils, silicone oils), gels (i.e. hydrogel)or any other biocompatible material known in the art may be used. In oneembodiment, the whole cover incorporates at least one additive. Inanother embodiment, only a portion of said tubular cover incorporatessaid at least one additive. In this embodiment, for example, the tubularcover incorporates at least one additive (e.g. a filler that will allowfor the transfer of energy (e.g. electricity) across a portion of thetubular cover) in at least one section and has at least one differentadditive, or no additive, in another section.

Another embodiment of the invention comprises methods of removing amedical device comprising providing an extraction force to said medicaldevice that comprises a tubular cover that everts upon extraction. Inone embodiment, said medical device comprises an implantabletransmission lead. In another embodiment, said implantable transmissionlead is for cardiac applications. In another embodiment, saidimplantable transmission lead is for neurological applications. Inanother embodiment, the tubular cover comprises ePTFE.

In another embodiment, said method of removing a medical devicecomprises separating said evertable tubular cover away from the medicaldevice and extracting said medical device. In another embodiment, saidmethod comprises separating the layers of said tubular cover, in a“double walled” construction, and extracting said medical device. Saidseparation of the tubular cover away from the medical device or fromitself, in a double walled construction, reduces friction between thetubular cover and said medical device or between the tubular covermaterial sliding across itself and, in turn, reduces the force requiredto remove the medical device. In one embodiment, said method separationof the tubular cover from said medical device or from itself comprisespumping a fluid (e.g. saline), gas (e.g. air), and/or another substance(e.g. gel or other lubricating agent) that will separate said tubularcover from said medical device or from itself. In another embodiment,said medical device comprises an implantable transmission lead. Inanother embodiment, said implantable transmission lead is for cardiacapplications. In another embodiment, said implantable transmission leadis for neurological applications.

Another embodiment of the invention comprises a method of making amedical device comprising, positioning a tubular cover over said medicaldevice such that said cover everts upon application of a longitudinalforce when removing said medical device from the body. In oneembodiment, said medical device comprises an implantable transmissionlead. In another embodiment, said tubular cover is positioned over saidtransmission lead such that said cover everts upon application of alongitudinal force when removing said lead from the body. In anotherembodiment, said implantable transmission lead is for cardiacapplications. In another embodiment, said implantable transmission leadis for neurological applications. In another embodiment, the tubularcover comprises ePTFE. Said methods of attaching the tubular cover tothe implantable transmission lead is described above.

Although the medical device exemplified for use with the tubular coverof the invention comprises an implantable transmission lead, the conceptof an evertable tubular cover can be applied equally well (and iscontemplated as part of the invention) to any indwelling device placedinto a body of an animal, e.g. a human, for a prolonged period of timeand which may be removed from said body. Examples include medicaldevices such as lead generators, arteriovenous (A-V) access catheters,peripherally inserted central (PICC) catheter lines, venous feedingcatheters, breathing tubes, feeding tubes, implanted sensing devices, orany medical device which is prone to tissue incorporation and the needfor atraumatic extraction.

One example of a medical device with an evertable tubular cover is avascular graft. Vascular grafts are well known in the art. There aremany instances when vascular grafts, such as dialysis grafts, must beremoved and/or replaced. Performance of some grafts decay after beingcannulated for some time. These grafts, now full of holes may allowpseudo aneurysms to occur, or may occlude due to increasingly turbulentblood flow. Many times, infection of the implant necessitates removal.It would be beneficial if removal could be achieved completely, easilyand atraumatically.

One embodiment of a vascular graft is disclosed in U.S. PatentApplication Publication U.S. 2009/0076587, which is incorporated hereinby reference for all purposes. Said vascular graft is disclosed in FIGS.10 and 11, as 1002, with the addition of an evertable tubular cover 1004of the invention. Said vascular graft disclosed in FIGS. 10 and 11 is astented vascular graft, which incorporates at least one stented section1006 onto a portion of its length. The at least one stent section can belocated at one end of said vascular graft and/or on both ends of saidvascular graft. The stented section is, in one embodiment, aself-expanding stent, although it may alternatively be a balloonexpandable stent or a combination thereof. Stent 1008 in stented section1006 is generally diametrically expandable tubular framework, typicallyof metal such as stainless steel or nitinol that is intended to providesupport to a body conduit when implanted by expansion to cause it tocontact the luminal surface of the body conduit. It has open spacesbetween adjacent framework elements of stent 1008. A conventionalvascular graft is defined herein as a tubular conduit capable ofconveying blood, without loss of blood through the wall of the vasculargraft (unless punctured or otherwise damaged).

The tubular cover of the invention may be attached to at least the anopposing end of said vascular graft by mechanical, rail or interferencefit, mechanical structures, heat bonding or by a biocompatible adhesiveor other securing means. Example adhesives are thermoplasticfluoropolymers, such as fluorinated ethylene propylene (FEP). Thetubular cover may also be formed as an integral part of the implantablethe vascular graft at manufacture, thereby not requiring any attachment.The tubular cover may also be attached to said vascular graft followingits manufacture as a separate component. In one embodiment, the tubularcover has “double walled” (as described above) construction wherein thecover is folded back on itself. Thus, when said vascular graftcomprising an evertable “double walled” tubular cover is extracted, thecover will slide over itself, thus reducing the force necessary toextract said vascular graft.

In one embodiment of the invention, a tubular cover 1004 is adhered tothe graft as shown in FIG. 11A (1108), while allowing said tubular coverto tear at specific sections where there is tissue attachment, tissueingrowth or another type of constraint that causes said tubular cover totear as the vascular graft is extracted. Other methods of attaching saidtubular cover of the invention are described above and are known in theart.

As shown in FIGS. 10 and 11, said stented vascular graft can comprise aninner tubular liner (i.e., a tubular element) 1102 that extendscontinuously in uninterrupted fashion between opposing ends of the graftand is made from polymeric materials typically used for conventionalvascular grafts such as polyester or ePTFE. This continuous innertubular liner provides a continuous luminal surface for blood contactthat is uninterrupted, or substantially uninterrupted, by seams orjoints. The length portion of the graft that does not include the stentpreferably has a second, outer layer 1104 of conventional vascular graftmaterial (i.e., a second tubular element) coaxially surrounding theinner layer, thus providing a thicker graft wall thickness in theunstented region. The greater graft wall thickness in the unstentedregion provides the desirable attributes of a conventional vasculargraft, including good bending properties with kink resistance, good hoopstrength and is readily sutured while providing good suture strength(resistance to tearing out of sutures). Additionally, for hemodialysisapplications, the greater wall thickness of the unstented region isanticipated to reduce time to achieve hemostasis following thewithdrawal of a dialysis needle from a penetration through the wall ofthe stented vascular graft. If desired, layers of reinforcing materialssuch as ePTFE film may be applied to the external surfaces of either orboth the inner and outer graft components and/or over the stent fordesired purposes such as to increase hoop strength or to aid in joiningcomponents. One such reinforcing material layer is depicted as 1106 inFIG. 11A. FIG. 11B also depicts at least a portion of the stentedsection 1006 (FIG. 10) of said stented graft in a body conduit 1102,such as the vasculature (e.g. a vein for an arteriovenous graft). Inthis case, as longitudinal force (shown buy arrow 1104) is applied, thetube everts, as shown by curved arrows 1108 and 1110.

Said everting cover can comprise a means of permitting tearing inspecific locations along the longitudinal axis of said tubular cover.Said means will allow the portions of the tubular cover to tearcircumferentially and evert as the vascular graft is extracted. In oneembodiment, said means comprise perforations being generally orientedcircumferentially around the tubular cover. In another embodiment, saidperforations are oriented in helical pattern around the tubular cover.In another embodiment, said means comprise weakened areas of materialthat will tear circumferentially and evert as the vascular graft isextracted. In these embodiments, said tubular cover will be secured (asdescribed above) to the vascular graft at strategic locations so that ifsaid tubular cover tears, said section will evert. In some embodiments,all the sections will tear and evert as the vascular graft is extracted.In other embodiments, only a few sections will tear and evert as saidvascular graft is extracted.

The tubular cover, 1004 may additionally be produced from very durable,thin ePTFE composites which would provide an additional benefit ofprotecting the vascular graft during the implantation process. A commontechnique to implant dialysis grafts is to create a tissue “tunnel” andpull or push the graft through the tract. The shear and torsion forcesduring implantation could be harmful to the implant. The thin,lubricious and durable tubular cover 1004 may aid in protecting theindwelling implant from these forces while also acting as an atraumaticremoval aid.

Said stented vascular graft may be used for a variety of intraluminalapplications, it is anticipated to be particularly useful as anarteriovenous graft for vascular access during kidney dialysis. Astented end of the graft is preferably intended to provide the distal,venous anastomosis. By eliminating the conventional sewn anastomosis atthe distal end of such dialysis grafts, it is anticipated that the rateof graft failure due to intimal hyperplasia at the outflow anastomosis(a common failure mode of these grafts) will be significantly reduced.

While it is shown that the stented vascular graft is provided with astent located at one end, it is apparent that both ends may be providedwith stents. Likewise, one or more stents may be provided at locationsbetween the ends of the graft. The stented vascular graft can be made ina variety of forms including various lengths and inside diameters. Itmay also be tapered along the length of the device so that the insidediameter is different at the opposing ends. Incorporation of taperedsections along the length of the device may be particularly desirablefor some dialysis graft applications. Dialysis grafts are often providedwith a smaller inside diameter at the arterial end than at the venousend. A tapered length section may be located closer to either end of thegraft, or the taper may exist as a uniform, gradual taper extendingbetween the graft ends.

The unstented portion may also be provided with reinforcing rings orspirals attached to the exterior surface; these exterior reinforcingcomponents may be made so as to be removable by a practitioner.Commercial vascular grafts of this type are available from W.L. Gore &Assoc., Flagstaff Ariz. 86003; see, for example, product no.SRRT06060080L. The unstented portion may also be provided with interiorreinforcing in a manner taught by U.S. Pat. No. 5,747,128 to Campbell etal. Likewise, particularly for hemodialysis applications, the unstentedportion may be provided with a layer of a self-sealing elastomer betweeninner and outer ePTFE layers in a fashion similar to the vascular grafttaught by Schanzer in U.S. Pat. No. 4,619,641. The unstented portion mayalso be provided with other means of rendering the porous graft materialnon-porous or less porous if desired, such as by the use of coatings ornon-porous or reduced porosity films applied to any portion or surfaceof either the inner or outer graft component. Coatings may also beapplied to fill or substantially fill microscopic void spaces betweenthe opposing graft surfaces. Although the Figures only depict a stentedvascular graft, it is contemplated as part of the invention that all theabove vascular grafts can comprise the tubular cover of the invention.

As with other vascular grafts, the stented vascular graft may beprovided with known therapeutic agents (e.g., any of variouspharmaceutic agents; anticoagulants such as heparin, etc.). WO02/26281A1provides a representative list of such agents, although the list is notintended to be limiting as to agents that might be used. These agentsmay be applied to the abluminal and/or luminal surfaces, and/or may beincorporated into the void space of the porous microstructure of thevascular graft tubing (e.g., ePTFE tubing). The application of theagents may be by any known means (e.g., such as coating) that aresuitable for attachment of the desired agent to the stented vasculargraft.

Thus, one embodiment of the invention comprises a medical devicecomprising a vascular graft, a tubular cover positioned over asubstantial portion of the outer surface of the vascular graft, whereinsaid tubular cover is configured to evert upon application of alongitudinal force to the stent-graft following implantation. Anotherembodiment of the invention comprises a tubular cover for a vasculargraft, said tubular cover comprising a material configured into atubular cover positionable over a substantial portion of an outersurface of a vascular graft and will evert upon application of alongitudinal force. In another embodiment, said vascular graft comprisesa stent. In another embodiment, said vascular graft comprises ePTFE. Inanother embodiment, the tubular cover comprises ePTFE. In anotherembodiment, said ePTFE comprises at least one additive. In anotherembodiment, said additive is a lubricating agent. In another embodiment,said additive elicits a bioactive response. In another embodiment, saidbioactive response is selected from the group consisting of ananti-inflammatory, an anti-microbial and an anti-proliferative response.In another embodiment, said tubular cover modulates the degree of tissueingrowth and/or adhesion. In another embodiment, said tubular coverresists tissue ingrowth and/or adhesion.

Another embodiment of the invention comprises a vascular graft having alength extending between opposing ends comprising, a first tubularelement extending for the length of the vascular graft, a first stentcoaxially affixed around a first length portion of the first tubularelement at one of said opposing ends, a second tubular element coaxiallyaffixed around a second length portion of the first tubular element thatdoes not include a stent, wherein the length of the second portion isgreater than the length of the first length portion; and a tubular coverpositioned over a substantial portion of the outer surface of thevascular graft, wherein said tubular cover is configured to evert uponapplication of a longitudinal force to the stent-graft followingimplantation. In one embodiment, one end of said second tubular elementis located adjacent to one end of said first stent. In anotherembodiment, said first tubular element provides an uninterrupted luminalsurface extending for the length of the vascular graft. In anotherembodiment, said first stent is a self-expanding stent. In anotherembodiment, said vascular graft comprises a therapeutic agent. Inanother embodiment, said therapeutic agent comprises herparin. Inanother embodiment, said heparin is on the luminal surface of saidvascular graft. In another embodiment, said vascular graft comprisesePTFE.

The following embodiments of the present invention will now be furtherdescribed by way of exemplary test methods and examples which are notintended to limit the scope of the invention in any manner.

EXAMPLES Test Methods Utilized in the Examples Withdrawal Force

The test apparatus is shown in FIG. 4.

Two silicone strips 600 and 620, 3.8 cm wide, 34.3 cm long, and 0.9 cmthick were cut from silicone foam rubber sheet stock (part #87485K75,McMaster Carr, Atlanta, Ga.). One strip 600 was placed on a flat surface680.

Next, a mandrel or mandrel plus polymeric tube 610 of an exampledescribed below was placed on top of strip 600. The second strip 620 wasplaced on top of the mandrel or mandrel plus polymeric tube.

A 4.45 cm wide, 34.29 cm long and 2.54 cm thick rectangular steel block630 having a mass of 3.09 kg was placed on top of the top silicone strip620. The curved end of the mandrel 610 extended beyond the length of thestrips and was attached to the hook 660 of a force gage 640 (Ametek,Accuforce III model, Ametek Corp, Paoli, Pa.). The force gage was boltedto a mounting plate 670 of a Minarik Controller 650 (Model No.WCG81596981, Minarik Corp., Glendale, Calif.).

The controller conditions were set to a traverse speed of 90 cm perminute. In this way, a longitudinal tensile force was applied to themandrel. The bare mandrel or mandrel plus polymeric tube was pulledbetween the silicone strips until the mandrel was completely free of thestrips. Peak force was recorded for each pull. Each test was repeatedthree times and the results were averaged.

Electrical Transmissivity and Release of Electrolytically Produced Gases

The test apparatus is shown in FIG. 8.

A 500 ml glass beaker 500 was filled with 500 ml of a 0.45% sodiumchloride (NaCl) solution 540. A covered coil 510 of Example 4 describedbelow was submerged in the solution.

An indifferent electrode 520 was submerged in the solution andpositioned as shown in FIG. 8, 50 mm 550 from the covered coil. Theindifferent electrode 520 was connected to the positive terminal of apulse generator 530 (Heartstart MRX, Philips Medical Systems, N.A.,Bothell, Wash.). The covered coil 510 was connected to the negativeterminal of the pulse generator by attaching both loose ends 570 of thecoil 510 to an alligator clip 580 on a lead 580 connected to the pulsegenerator.

Electrical pulses of varying energy levels were applied between thecovered coil and the indifferent electrode and the impedances measured.The coil cover was removed from the test apparatus and visuallyinspected for mechanical disruption.

Material Properties

Material thickness was measured using a digital thickness gauge with a1.3 cm diameter foot (Model ID-C112E, Mitutoyo, Aurora, Ill.). Fivemeasurements were made and averaged. Length and width of the materialwere measured using a metal ruler. Five measurements were made andaveraged.

Material was weighed using a precision analytical balance (model PM400,Mettler-Toledo, Inc, Columbus, Ohio). Maximum load was measured using atensile test machine equipped with a 10 kg load cell (Model 5564,Instron, Grove City, Pa.). The gauge length was 2.5 cm and thecross-head speed was 25 mm/minute.

Tensile test samples were 7.60 cm×2.50 cm. Longitudinal tensile testmeasurements were taken in the length direction of the material andtransverse tensile test measurements were taken in the directionorthogonal to the length direction. The longitudinal and transversematrix tensile strengths (MTS) were calculated using the followingequation:

${{Matrix}\mspace{14mu} {Tensile}\mspace{14mu} {Strength}} = \frac{\left( {\sigma \mspace{14mu} {sample}} \right)*\left( {\rho \; {PTFE}} \right)}{\left( {\rho \mspace{14mu} {sample}} \right)}$

Where:

-   -   ρ PTFE=2.2 grams/cc    -   σ sample=(Maximum Load/Width)/Thickness    -   ρ sample=(Mass/Area)/Thickness

Density was calculated using the formula, density=mass/volume. Theaverage of three values was reported.

The permeability of the material was measured using a GurleyPermeability instrument, equipped with a 2.5 cm orifice area (Model4110, Teledyne Gurley, Troy Mich.). Values were recorded as the time inseconds for 300 cc of air to flow through a 6.45 cm² sample. The averageof three sample measurements was used.

The maximum pore size of three samples of material was measured as theisopropyl alcohol bubble point using a pressure regulator bubble pointtester (Model LC-APOK, Salt Lake City, Utah). The three values wereaveraged and reported.

Ethanol bubble point (EBP) is the minimum pressure required to force airthrough an ethanol-saturated material (such as ePTFE). Raising thepressure slightly should produce steady streams of bubbles at manysites. Thus, the measurements are not biased by artifacts such aspuncture holes in the material. Ethanol bubble point is inverselyrelated to pore size; lower values of EBP indicate larger pores. It isbelieved that EBP can be assumed to be independent of the length of thepath that the air travels through the article. In other words, it isbelieved that EBP provides a characterization of pore size that is notunacceptably dependent on the dimensions of the tested article. Note thedata below is based on an isopropyl (not ethanol) bubble point test.

Comparative Example 1 Bare Mandrel

Referring to FIG. 5, a straight, solid stainless steel mandrel 730measuring 35.60 cm long and 0.25 cm diameter (available from New EnglandPrecision Grinding, Holliston, Mass.) was obtained. A 0.25 mm deep and1.15 mm wide groove 710 was cut around the circumference of the mandrel730, 4.73 mm from one end. A chamfer 720 was formed at the grooved endof the mandrel. 15 mm of the other end of the mandrel was bent into ahook shape 700 as shown in FIG. 5.

The force to withdraw the mandrel from between the two silicone stripswas measured as described above. The average of the peak forces was 4.00kg, as seen in Table 1.

Comparative Example 2 Single Walled Tube Attached at Curved End ofMandrel

Expanded polytetrafluoroethylene (ePTFE) material was obtained with thefollowing properties: Thickness=0.0025 microns, Width=6.5 cm,Length=25.4 cm, Mass/Area=2.5 g/m2, Longitudinal MTS=558.5 MPa,Transverse MTS=29.6 MPa, Gurley number=2 sec/6.45 cm2/100 cc, and IPABubble Point=0.23 MPa.

A straight, solid stainless steel mandrel measuring 35.6 cm long and0.25 cm diameter (available from New England Precision Grinding,Holliston, Mass.) was obtained. Six and a half layers of the materialwere wrapped about the circumference of the mandrel such that the lengthdirection of the material was oriented in the longitudinal direction ofthe mandrel. Consequently, a single longitudinally-oriented seamresulted. In this way, the higher strength direction of the ePTFEmaterial was oriented along the longitudinal axis of the mandrel. Asoldering iron (Model Weller WESD51, Cooper Industries, Houston, Tex.),set at 343.3° C. was passed along the full length of the seam.

The mandrel plus ePTFE was placed in a forced air oven (Grieve ModelNT1000, single phase, Grieve Corp, Round Lake, Ill.) set to 370° C. for7 minutes then removed from the oven and allowed to cool. An ePTFE tuberesulted. The tube was removed from the mandrel.

A curved end mandrel, as described in Comparative Example 1 wasobtained. The ePTFE tube was slid over the grooved end of the curved endmandrel. A drop of Loctite 495 adhesive (Henkel Australia PTY. LTD., 1Clyde St., Sliverwater NSW229, Australia) was applied to the edge of thetube nearer the curved end. A 1.0 cm long piece of 0.32 cm diametershrink tube (Part no. HS-101, Insultab, 45 Industrial Parkway, Woburn,Mass.) was slid over the mandrel to cover the tube edge to which theadhesive had been applied. The shrink tubing was heated at 211.6° C. for10 seconds in order to assist the bonding of the tube to the mandrel.This was done using a thermal box (Balloon Development Station #210A,Beahn Designs, Los Gatos, Calif.), with air output set at 25 standardcubic feet per hour (SCFH). The adhesive was allowed to cure underambient conditions for 0.5 hours.

The force to withdraw the mandrel plus the ePTFE tube from between thetwo silicone strips was measured as described above. The average of thepeak forces was 3.72 kg, as seen in Table 1.

Example 3 Single Walled Tube, Everting as Removed

An ePTFE tube was constructed and placed over a curved ended mandrel asdescribed in Comparative Example 2.

Referring to FIG. 6A, the ePTFE tube 330 was attached to the mandrel 730at the groove 710 using one loop of suture 310 (CV5, W.L. Gore andAssoc., Flagstaff, Ariz.) and secured with a square knot. The knot waspressed into the groove to minimize profile.

The mandrel plus ePTFE tube were then tested as described above.

In this case, as longitudinal force (shown buy an arrow in FIG. 6B) wasapplied, the tube everted, as shown by curved arrows 320 as the mandrelwas withdrawn. The average of the peak forces was 1.23 kg, as seen inTable 1.

Example 4 Double Walled Tube, Everting as Removed

An ePTFE tube was constructed as described in Comparative Example 2,except that it was made twice as long (50.8 cm).

Referring to FIG. 7A, a curved end mandrel 730, as described inComparative Example 1, was obtained. One half of the tube was placedover a mandrel and attached at the curved end using an adhesive andshrink tube, as described in Comparative Example 2. The other half ofthe tube was then folded back over the first half of the tube, towardthe curved end of the mandrel 730, creating two layers 400 and 410 oftube material over the length of the mandrel 730 as shown in FIGS. 7Athrough 7C. Also shown in FIGS. 7A through 7C is the shrink tube 450,first tube end 430 and second tube end 440.

The mandrel was then withdrawn as indicated by the straight arrow inFIGS. 7B and 7C. The tube everted, as shown by curved arrows 460 as themandrel was withdrawn. The force to withdraw the mandrel plus the ePTFEtube from between the two silicone strips was measured as describedabove. The average of the peak forces was 1.20 kg, as seen in Table 1.

Example 5 Single Walled Tube HDPE Tube Everts as Removed

High density polyethylene (HDPE) material was obtained from a can liner(part #HR171806C, Waverly Plastics, Waverly, Iowa). The HDPE filmmaterial was 6.5 cm wide, 25.4 cm long and 0.006 mm thick. A tube wascreated by longitudinally wrapping the HDPE material film on a straight,solid stainless steel mandrel as described in Comparative Example 2.

The only difference from that example was that bonding of the seam wasachieved through the use of silicone adhesive (Adhesive Silicone Type A,Part no. MED1137, NuSil Technology, Carpenteria, Calif.). The adhesivewas allowed to cure under ambient conditions overnight.

The HDPE tube was removed from the mandrel. The HDPE tube was mounted ona curved end mandrel and attached to the mandrel as described in Example3. The force to withdraw the mandrel plus the HDPE tube from between thetwo silicone strips was measured as described above. The HDPE tubeeverted upon withdrawal. The average of the peak forces was 1.12 kg, asseen in Table 1.

TABLE 1 Withdrawal forces Comparative Comparative Example 1 Example 2Example 3 Example 4 Example 5 4.00 kg 3.72 kg 1.23 kg 1.20 kg 1.12 kg

Example 6 Electrical Transmissivity and Release of ElectrolyticallyProduced Gases

A silver plated copper rod, 2.4 mm in diameter and 20.3 cm in length wasobtained (Phelps Dodge Industries, Co., Phoenix, Ariz.). A coil wasproduced by wrapping the center portion of the rod with a 0.15 mmdiameter wire having a 25% silver core and anickel-cobalt-chromium-molybdenum outer layer. (MP 35N DFT, 25% Ag, FortWayne Metals, Fort Wayne, Ind.). The coil was wrapped without gapsbetween the wires. The finished coil measured 6.3 mm in length. About3.0 cm of wire was left unwrapped and loose on each end of the coil.

Expanded polytetrafluoroethylene (ePTFE) material with the propertiesgiven in Comparative Example 2 was obtained. The material was wrappedaround the coil as described in Comparative Example 2. The material wasseam sealed as described in Comparative Example 2. The rod and coil plusePTFE assembly was placed in a forced air oven (Grieve Model NT1000,single phase, Grieve Corp, Round Lake, Ill.) set to 370° C. for 10minutes then removed from the oven and allowed to cool.

One end of the silver plated copper rod was clamped into a vice and theother was gripped with a set of vice grips and pulled in the lengthdirection. This reduced the diameter of the rod underneath the coilallowing it to slip off the rod.

Referring to FIG. 8, the covered coil 510 was then chemically treated asfollows. It was soaked in isopropyl alcohol (IPA) at ambient temperature(about 21° C.) for 15 minutes. Then it was immediately transferred to asolution of 2.0% polyvinyl alcohol (PVA) and de-ionized water andallowed to soak at ambient temperature for 70 minutes. The covered coilwas rinsed for 20 minutes in de-ionized water at ambient temperature andthen soaked for 50 minutes in a solution of 2% gluteraldehyde, 1%hydrochloric acid (HCL) and de-ionized water, at ambient temperature.The covered coil was rinsed in de-ionized water at ambient temperaturefor 2 hours and then allowed to dry in ambient air. It was then placedinto the test apparatus as shown in FIG. 8 and allowed to soak for 1minute. The loose wires at each end 570 were attached to the pulsegenerator lead 580 via a lead 580.

The pulse generator was set to deliver 30 J electrical pulses and apulse was delivered between the covered coil and the indifferentelectrode. This was repeated 3 times at 30-40 second intervals. Thepulse generator was then set to the following conditions: 50 J and 50seconds between pulses; 70 J and 18 seconds between pulses; 100 J and 11seconds between pulses; 150 J and 15 seconds between pulses; 150 J and18 seconds between pulses.

The electrical continuity of the covered coil in the test apparatus wasretained for the duration of the test as evidenced by the visualdetection of gas microbubble electrolysis emitted from the length of thecovered coil during each pulse and measured impedances for each pulseremaining in a narrow range of between 26.2-27.7 ohms.

The covered coil was removed from the test apparatus, dried andinspected using a 30× stereo microscope. No mechanical disruption (tearsor holes) were noted in the cover of the coil. No mechanical damage tothe coil was observed.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A medical device comprising: a vascular graft; a tubular coverpositioned over a substantial portion of the outer surface of thevascular graft; wherein said tubular cover is configured to evert uponapplication of a longitudinal force to the stent-graft followingimplantation.
 2. The device of claim 1, wherein said vascular graftcomprises a stent.
 3. The device of claim 1, wherein said vascular graftcomprises ePTFE.
 4. The device of claim 1, wherein the tubular covercomprises ePTFE.
 5. The device of claim 4, wherein said ePTFE comprisesat least one additive.
 6. The device of claim 5, wherein said additiveis a lubricating agent.
 7. The device of claim 5, wherein said additiveelicits a bioactive response.
 8. The device of claim 7, wherein saidbioactive response is selected from the group consisting of ananti-inflammatory, an anti-microbial and an anti-proliferative response.9. The device of claim 4, wherein said tubular cover modulates thedegree of tissue ingrowth and/or adhesion.
 10. The device of claim 4,wherein said tubular cover resists tissue ingrowth and/or adhesion. 11.A tubular cover for a vascular graft, said tubular cover comprising amaterial configured into a tubular cover positionable over a substantialportion of an outer surface of a vascular graft and will evert uponapplication of a longitudinal force.
 12. The tubular cover of claim 11,wherein the material is ePTFE.
 13. The tubular cover of claim 12,wherein said ePTFE comprises at least one additive.
 14. The tubularcover of claim 13, wherein said additive is a lubricating agent.
 15. Thetubular cover of claim 13, wherein said additive is a antimicrobial. 16.The tubular cover of claim 13, wherein only a portion of said tubularcover comprises an additive.
 17. The tubular cover of claim 13, whereinsaid additive elicits a bioactive response.
 18. The tubular cover ofclaim 17, wherein said bioactive response is selected from the groupconsisting of an anti-inflammatory, an anti-microbial, and ananti-proliferative response.
 19. The tubular cover of claim 12, whereinsaid tubular cover modulates the degree of tissue ingrowth and/oradhesion.
 20. The tubular cover of claim 12, wherein said tubular coverresists tissue ingrowth and/or adhesion.
 21. A vascular graft having alength extending between opposing ends comprising: a first tubularelement extending for the length of the vascular graft; a first stentcoaxially affixed around a first length portion of the first tubularelement at one of said opposing ends; a second tubular element coaxiallyaffixed around a second length portion of the first tubular element thatdoes not include a stent, wherein the length of the second portion isgreater than the length of the first length portion; and a tubular coverpositioned over a substantial portion of the outer surface of thevascular graft, wherein said tubular cover is configured to evert uponapplication of a longitudinal force to the stent-graft followingimplantation.
 22. The vascular graft of claim 21, wherein one end ofsaid second tubular element is located adjacent to one end of said firststent.
 23. The vascular graft of claim 21, wherein said first tubularelement provides an uninterrupted luminal surface extending for thelength of the vascular graft.
 24. The vascular graft of claim 21,wherein said first stent is a self-expanding stent.
 25. The vasculargraft of claim 21, wherein said vascular graft comprises a therapeuticagent.
 26. The vascular graft of claim 25, wherein said therapeuticagent comprises herparin.
 27. The vascular graft of claim 21, whereinsaid vascular graft comprises ePTFE.
 28. The vascular graft of claim 21,wherein said tubular cover comprises ePTFE.
 29. The vascular graft ofclaim 28, wherein said ePTFE comprises at least one additive.
 30. Thevascular graft of claim 29, wherein said additive is a lubricatingagent.
 31. The vascular graft of claim 29, wherein said additive elicitsa bioactive response.
 32. The vascular graft of claim 31, wherein saidbioactive response is selected from the group consisting of ananti-inflammatory, an anti-microbial and an anti-proliferative response.33. The vascular graft of claim 28, wherein said tubular cover modulatesthe degree of tissue ingrowth and/or adhesion.
 34. The vascular graft ofclaim 28, wherein said tubular cover resists tissue ingrowth and/oradhesion.